In assembling electronic components and modules, inserts, spacers and standoffs have been often used. The attachment of components and parts has been accomplished by screws, spring clips, clamps and other such devices. In a chassis for holding electronic components, space for the manual manipulation of parts and tools often is an issue.
Captive screws and captive fasteners are devices used to fasten two components together, where the fastener remains with one of the components when loosened. Typically a captive screw is “caught” by the component it remains with by a flange, a ferrule, a spring clip or the like, which structure prevents the total removal of the captive fastener from that component. The usefulness of captive fasteners is that they do not get lost or fall out of the associated component before and during assembly.
This feature has become very useful in the assembly and removal of components associated from electronic module boards, peripheral component interconnect boards (PCI boards), and printed circuit boards (PC boards), and in the environment of the chassis for housing these boards.
Modern large scale integrated (LSI) circuits, microprocessors, microchips and other integrated circuit devices (IC chips) generate a substantial amount of heat, especially when operating at very high frequencies. Such heat generation can amount to 10's of watts and even 100's of watts of heat per hour. It has become imperative to mount heat sinks on these IC chips to dissipate as much heat as possible. In such instances the heat sink is mounted to the board or to a mounting frame which in turn is mounted to the board on which the IC chip is also mounted.
Spring clips have been used to hold heat sinks to IC chips on PC boards. However, these clips are sensitive to vibration, often interfere with the heat transfer fins on the heat sink and are often hard to positively snap into place and to release.
Captive screws have provided and improvement over heat sink clips. Two or four captive screws are used and engage respective flanged corners of a heat sink. These captive screws have threaded ends which usually engage a threaded ferrule or threaded bushing mounted into a hole through the PC board. They also require a ferrule or bushing though the heat sink's flange through which they extend.
A captive screw may use a slip ring, annular flange, or projecting shoulder positioned on the captive screw at a location below the heat sink flange's surface. This projecting structure prohibits the captive screw from being withdrawn out of the heat sink and thereby holds the fastener captive on the heat sink. Captive screws are generally driven (tightened and loosened) by tool engagement with their head. Typically, captive screws have Phillips, slotted, or TORX heads requiring appropriate screw drivers.
Oftentimes a sheet of compressible elastomeric heat transfer polymeric material is used between the top surface of the IC chip and the bottom of the heat sink. This heat transfer interface material takes up for any surface irregularities in the mating IC chip and heat sink.
Captive screws for IC chip heat sinks with heat transfer polymeric sheeting have incorporated spring tie-down designs where the tie-down force exerted by the captive screw is governed by the spring force of a compresses spring. This structure permits the heat sink to “float”, i.e., move through expansion and contraction as the IC chip temperature changes.
As the chassis for electronic modules is made smaller with a smaller foot print, and as more boards are crowded into tightly spaced racks in a chassis, the size and position of heat sink tie-down screws, including captive screws, becomes an issue. Moreover, Phillips, slotted and even TORX heads can round out with poor tool alignment. The use and installation of board mounted receiving threaded bushings or ferrules and of heat sink mounted ferrules adds to the cost of the securement hardware. Alignment of the heat sink assembly when aligning the heat sink screws with the board mounted receiving threaded bushing or ferrules generally requires two hands and some lateral movement. This lateral movement can jeopardize the integrity of the printed circuit coating on the board, and miss-align the interface polymer heat transfer pad. This makes the removal and reinstallation of heat sinks in tight quarters difficult.
To improve the ease of alignment of a heat sink and to assure proper positioning thereof, permanent board mounted studs have been proposed. These studs can be threaded for securing a fastener thereto and can also act as alignment pins during the removal and the reinstallation of a heat sink. What is desired is a nut-type structure for use with board mounted studs for securing a heat sink to an IC chip.
The objective of the present invention is to provide a captive nut structure for use with IC chip heat sinks, which captive nut structure remains with the heat sink when it is removed.
A second objective is to provide spring tie-down which will permit the heat sink to float with temperature changes.
A third objective is to minimize the manufacturing costs of the captive nut structure and to minimize the number of components thereof